An EL device has high visibility owing to self-emission of light, and has excellent impact resistance owing to a complete solid device. Due to these characteristics, there have been proposed a variety of inorganic EL devices for which inorganic compounds are adapted as a light-emitting material and a variety of organic EL devices for which organic compounds are adapted as a light-emitting material. Above all, developments of organic EL devices are actively underway for obtaining organic EL devices having higher performance, since the organic EL devices permit a decrease in drive voltage to a great extent as compared with inorganic EL devices.
In the basic constitution of an organic EL device, the organic EL device has a structure in which an anode, a light-emitting layer and a cathode are consecutively laminated, and the above organic EL device is formed on a substrate in many cases. The anode and the cathode may be reversed in position. In some cases, further, a hole-transporting layer is provided between the anode and the light-emitting layer, and an electron-injecting layer is provided between the cathode and the light-emitting layer, for improving the performance. The light-emitting layer is generally formed of one or a plurality of organic light-emitting materials, while it is sometimes formed of a mixture of an organic light-emitting material with a hole-transporting material and/or an electron-injecting material.
Further, in a pair of electrodes (anode and cathode) constituting the organic EL device, the electrode positioned on a surface through which light comes out is formed of a transparent or semi-transparent film for improving the light emission efficiency and due to a constitution as a surface light-emitting device. The other electrode (to be sometimes referred to as "opposite electrode" hereinafter) positioned opposite to the surface through which light comes out is formed of a specific metal thin film (thin film of metal, alloy or a mixture of metals).
The organic EL device having the above constitution is a current-driven light-emitting device, and it is required to apply a high electric current between the anode and the cathode for performing light emission. As a result, the device generates heat when the device is emitted, and when the device has oxygen or water around it, the oxygen and the water promote the oxidation of materials forming the device to degrade the device. In the typical degradation of the device by the oxidation and water, a dark spot occurs and grows. The dark spot refers to a fault point of light emission. As the oxidation of the materials forming the device proceeds with the driving of the organic EL device, the existing dark spot grows, and eventually, the dark spot spreads over the entirety of the light-emitting surface.
For preventing the above degradation, a variety of methods have been hitherto proposed. For example, for effectively removing heat which the device generates when the device is emitted, JP-A-4-363890 discloses a method in which an organic EL device is held in an inert liquid compound of liquid fluorinated carbon. As a method of removing water which is one of those which cause the degradation, JP-A-5-41281 discloses a method in which an organic EL device is held in an inert liquid compound prepared by incorporating a dehydrating agent such as synthetic zeolite into liquid fluorinated carbon (specifically, the same as the liquid fluorinated carbon disclosed in the above JP-A-4-363890). Further, JP-A-5-114486 discloses a method in which a heat-radiating layer encapsulating a fluorocarbon oil (specifically, included in the liquid fluorinated carbon disclosed in the above JP-A-4-363890) is formed on at least one of the anode and the cathode and heat generated at a time of driving the device is radiated through the heat-radiating layer to extend the light emission life of the device.
The degradation caused by water takes place in an inorganic EL device as well. As a method of preventing the degradation of the inorganic EL device, there is a method in which an inorganic EL device is encapsulated between a pair of glass substrates while providing a predetermined space and the space is charged with a protective liquid. U.S. Pat. No. 4,446,399 discloses a method in which silicone oil or silicone grease is used as the above protective liquid. U.S. Pat. No. 4,810,931 discloses a method in which a liquid obtained by degassing a perfluorinated inert liquid (specifically, the same as, or similar to, the liquid fluorinated carbon disclosed in the above JP-A-4-363890) under conditions of a liquid temperature of approximately 90 to 120.degree. C. and an ambient pressure of about 10 Torr is used as the above protective liquid.
However, the occurrence and the growth of a dark spot in an organic EL device cannot be fully prevented even if the above conventional methods on the organic EL device are relied upon. The reason therefor is assumed to be as follows.
For preventing the occurrence and the growth of the dark spot, it is one of useful means to utilize liquid fluorinated carbon for removing the heat which the organic EL device generates when the device is emitted or to use a dehydrating agent for removing water which infiltrates the organic EL device during or after the organic EL device is encapsulated. However, studies by the present inventors show that the liquid fluorinated carbon and the fluorocarbon oil greatly dissolve gases, and not water which externally infiltrates but oxygen dissolved in the liquid fluorinated carbon or the fluorocarbon oil has a great influence on the occurrence and the growth of the dark spot. For example, perfluoroamine (Fluorinert (trade name), supplied by Sumitomo-3M Co., Ltd.) contains dissolved air in a maximum amount of 22 milliliters per 100 milliliters thereof (dissolved oxygen concentration 63 ppm). It is therefore difficult to fully prevent the occurrence and the growth of the dark spot by the above-described conventional methods on the organic EL device.
On the other hand, when the method of using a silicone oil or silicone grease as a protective liquid, among the above conventional methods on an inorganic EL device, is applied to the organic EL device, there is caused a problem that the organic EL device is degraded or broken by the silicone oil or the silicone grease. Further, when the method of using, as a protective liquid, a perfluorinated inert liquid which is degassed under the above-specified conditions is applied, it is difficult to fully prevent the occurrence and the growth of the dark spot in the organic EL device.
It is an object of the present invention to provide a method of encapsulating an organic EL device, in which the growth of the dark spot in the organic EL device can be firmly prevented, and an encapsulated organic EL device in which the growth of a dark spot is well prevented.